1. Field of the Invention
This invention relates to a frequency adjustment method and a frequency adjustment device which can suitably be used for a communication system, such as a wireless LAN which requires high speed frequency synchronization.
2. Description of the Related Art
For communication systems adapted to transmit information by modulating the carrier wave, the transmitter and the receiver are required to make their respective carrier wave frequencies coincide with each other. Generally, however, it is difficult for the transmitter and the receiver to keep their carrier wave frequencies accurately agreeing with each other. For this reason, a carrier wave adjustment operation is conducted at the receiver before it starts for an operation of communication.
Recently, wireless LANs have been put to practical use to allow personal computers to wirelessly communicate with each other and free them from positional restrictions imposed on them for operation. The wireless LAN typically uses a modulation method which is referred to as OFDM (orthogonal frequency division multiplexing), which requires high speed frequency synchronization. With OFDM, a receiver which receives a signal from a transmitter needs to synchronize its carrier wave frequency with which of the received signal at high speed. Generally, a method of coarsely adjusting the reception frequency of the receiver and then finely adjusting it is used for the purpose of frequency synchronization (see, Japanese Patent laid open (KOKAI) No. 2001-177436).
The above-described method of coarsely adjusting the reception frequency and then finely adjusting it in order to make it coincide with the target frequency is advantageous from the viewpoint of range and accuracy of adjustment. On the other hand, the method requires a relatively long period of time for frequency adjustment because the deviation of the reception frequency has to be gauged after the completion of the coarse adjustment operation and a fine adjustment operation has to be performed on the basis of the gauged deviation. The system such as wireless LAN (IEEE 802.11a) needs to complete automatic frequency control (AFC) for a short time. Thus, the time which AFC requires needs to be made as short as possible.
In order to quickly complete both the coarse adjustment operation and the fine adjustment operation, it is necessary to start the former operation immediately after the signal reception. However, the signals arranged at the top of a packet in a region referred to as preamble which is to be transmitted before transmitting data signals are apt to be subjected to frequency fluctuations due to noise and therefore it has been difficult to accurately detect the amount of the gap between the frequency of the received signal and which of the corresponding frequency of the receiver. Thus, there is a demand for a method and a device for frequency adjustment which can effectively suppress the influence of noise and precisely and reliably carry out a frequency adjustment operation.